Gell guards were blob-like creatures created by Omega, trapped in a black hole domain made of anti-matter. They had the ability to travel from Omega's anti-matter universe into that of matter.
They were unaffected by gunfire and explosions and could use their claws as weapons to create explosions of their own.
Under Omega's command they attacked UNIT HQ. They killed a number of soldiers, captured the Third Doctor and his friends, and took them to Omega. (TV: The Three Doctors)
The Gell guards were one of the species referenced in Theoretical Monsters: A Credibility Test. (PROSE: Interference - Book One)
Behind the scenes
The name "gell guard" does not appear onscreen. It first appears in the Doctor Who universe in Alien Bodies.